1. Field of the Invention
The present invention relates to output buffers with circuitry to reduce ground bounce and increase circuit operation speed. More particularly, the present invention relates to output buffers for programmable array logic (PAL) devices.
2. Description of the Related Art
Attempts to achieve higher speed from output buffers has resulted in sacrificing output signal distortion due to ground bounce or output signal ringing.
Ground bounce has two outward manifestations: First, with all inputs of a chip referenced to chip ground, if the ground bounce is high enough, 2.4 V for instance, an input at a TTL high can momentarily appear to be a low. Second, an output which is in a low state can momentarily bounce to a high state because the output is effectively connected to chip-ground through the pull-down output transistor.
FIG. 1 illustrates the ground bounce effect with a typical prior art output driver consisting of two output buffers 100 and 102 connected to a common ground lead 108. Assume initially that pull down transistor 104 of buffer 100 is off making output 1 high while pull down transistor 106 is on making output 2 low. When transistor 104 turns on switching output 1 from high to low with transistor 106 remaining on, the increase in current through the ground lead 108 results in a ground bounce that shows up on output 2. Output 2 may thus momentarily appear to be in a high state.
As can be seen from FIG. 1, ground bounce is an undesirable effect often associated with high speed output buffers used on devices containing multiple output buffers such as the AmPALCE22V10 manufactured by Advanced Micro Devices, Inc.
FIG. 2 shows a block diagram for the AmPALCE22V10 with output buffers 200(a-j). Output buffers 200(a-j) receive a signal from programmable array logic through output logic macrocells 202. The output logic macrocells 202 allow selection of whether the output is registered or combinatorial. When a number of output buffers, such as buffers 200(a-j), are hooked to capacitive loads and are switched from high to low simultaneously, the current through the ground lead will increase by an amount equal to the slew-rate times the capacitive load per output times the number of outputs being switched. Ground bounce is equal to the rate of change of this current times the inductance of the ground lead.
In prior art PAL devices ground bounce has been controlled by limiting the slew-rate of the output buffer. In fact, the slew-rate is programmable on the AmPALCE22V10 as well as many other PAL devices. However, by limiting the slew-rate, performance is sacrificed because the lower slew-rate will slow the part down and thus increase propagation delay.